Method of and apparatus for conveying molten metals while providing heat thereto

ABSTRACT

The invention relates to a method of and apparatus for providing heat to a molten metal flowing through metal-conveying apparatus. The apparatus includes a molten metal-conveying channel, an enclosure for receiving and circulating combustion gases while preventing entry of the gases into said channel, a heat-conductive body of material separating at least part of the channel from the enclosure; and a combustion device for generating combustion gases and delivering the gases to the enclosure. Heat from the combustion gases is used to heat molten metal held in the channel, while preventing contact between the combustion gases and the molten metal. The body of material may be a trough used to form the channel, a tube for conveying the molten metal, or a tube acting as the enclosure, or the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority right of provisional U.S. patentapplication Ser. No. 60/876,045 filed Dec. 19, 2006 by applicants namedherein.

BACKGROUND OF THE INVENTION

I. Field of the Invention

This invention relates to apparatus used for conveying molten metalsduring casting operations and the like. More particularly, the inventionrelates to apparatus for and methods of providing heat to molten metalsconveyed in such apparatus to prevent metal freeze-up, undue cooling, orsimilar effects, during the passage through such apparatus.

II. Background Art

It is common practice during metal casting operations to cause a moltenmetal to flow through an elongated trough (sometimes called a launder),for example from a melting furnace to a casting mold. Such troughs aremade of a material that can resist exposure to the molten metal for areasonable period of time without undue damage, and the conditions mustbe such that the metal does not cool below its freezing temperature(solidus) before it reaches its destination. When troughs of this kindare fairly short, fast-flowing (e.g. relatively steeply inclined) or ofrelatively small metal-holding capacity, there is little risk of metalfreezing. Recently, however, various new practices have made itnecessary to provide troughs of greater capacity, greater length and/orslower flow, particularly in the aluminum treatment arts. For example,U.S. Pat. No. 5,527,381 to Peter D. Waite, et al., which issued on Jun.18, 1996, discloses a method of treating a molten metal with a gas toremove dissolved hydrogen and other impurities as the metal flowsthrough a trough or launder. The treatment can be made more thorough ifthe trough is of large metal-holding capacity and the metal is caused toflow at a slow rate of throughput. Similarly, it is now possible toco-cast different molten metals to form a single ingot by direct-chillcasting, and the molten metal used for a cladding layer of such an ingotis generally cast in much smaller amounts than the molten metal used fora core layer, so that the metal for the cladding layer must flow moreslowly to the casting apparatus than the metal for the core.Additionally, molten metal is sometimes filtered through ceramic foamfilters to remove solid particles, and the use of such filters may slowthe flow of molten metal through a trough. Consequently, in applicationssuch as these and others, the risk of metal solidification (or unduecooling) in the trough is increased.

One way of eliminating the risk of metal solidification is to heat themetal in the trough or the trough itself. Metal in the trough can beheated by directing a flame onto the upper surface of the metal as itflows through the trough, but this has the disadvantage that oxidationof the metal at the surface is thereby accelerated, particularly if themetal is aluminum or an aluminum alloy. Heating of the trough can becarried out by providing electrical heaters on or adjacent to the innersurface of the trough, but generally such heaters are slow to transferheat to the metal and are therefore not always very effective inapplications of this kind.

Two patents illustrate the kind of approach taken in the past. U.S. Pat.No. 5,744,093 to John A. Davis, which issued on Apr. 28, 1988, disclosesthe provision of a covered trough to provide increased insulation. Gasesemerging from the trough are drawn into a plenum, and heat may beintroduced above the metal by means of a burner arrangement penetratingthe trough cover. Combustion gases from the burner are then withdrawnfrom the space above the metal by being drawn into the plenum.

U.S. Pat. No. 3,942,473 which issued on Mar. 9, 1976 to Charles M.Chodash is concerned with the accretion of copper and provides anenclosed trough having a covered head space above the metal channel. Themetal is kept at an elevated temperature either by providing radiantheaters in the head space or by directing gas flames onto the upper andlower surfaces of the trough.

There is a need for improvement of the heating of metal-conveyingtroughs, particularly for troughs of large capacity and/or slowthroughput, and particularly for apparatus intended for use withaluminum and aluminum alloys.

SUMMARY OF THE INVENTION

In exemplary aspects, a method and apparatus are provided for providingheat to a molten metal flowing through a metal conveying apparatus. Hotcombustion gases, generated by a burner or the like, are used to heat aheat-conductive material that comes into contact with the molten metal.However, the hot combustion gases are kept out of contact with themolten metal and are used to heat the metal solely by conduction throughthe refractory material. The heat-conductive material may be used toform a section of a trough, a channel element, or just a part of atrough or channel, or as an insert or body contacting the molten metal.The gases brought into contact with the heat-conductive material areconfined within one or more enclosures that allow the gases to flowthrough the apparatus in the form of a stream while preventing contactof the combustion gases with isolated the molten metal (and preferablyalso the external atmosphere surrounding the apparatus).

One exemplary embodiment provides a molten metal-conveying apparatus,comprising a molten metal-conveying channel, an enclosure for receivingand circulating combustion gases while preventing entry of said gasesinto said channel, a heat-conductive body of material separating atleast part of said channel from said enclosure, and a combustion devicefor generating combustion gases and delivering said gases to saidenclosure. In use, heat from said combustion gases is transferred tomolten metal held in said channel through said body of heat conductivematerial. Hence, the molten metal is heated by the combustion gases, butthe gases are kept out of direct contact with the molten metal in thechannel.

The heat-conductive body of material may form an elongated element (witha metal-contacting surface defining the channel and another surfacecontacting the combustion gases, e.g. an outside surface of theelongated element). In such a case, the elongated element may be anopen-topped trough section or an enclosed tube or tubes. Alternatively,the heat-conductive body of material may be separate from an elementdefining the channel, e.g. it may be a tubular member extending into thechannel formed in an elongated element.

In another exemplary embodiment, the invention provides a molten metalconveying trough apparatus. The apparatus includes a molten metalconveying trough section having an upper end and an outer surfaceextending around the trough section from the upper end. An enclosure atleast partially encloses the outer surface of the trough section, andthe enclosure contains at least one chamber adjacent to the outersurface. An entrance into the chamber, or an entrance into each chamberwhen there is more than one, is provided through which hot combustiongases are introduced into the or each chamber. An exit from the or eachchamber is also provided through which the hot combustion gases areremoved after flowing as a stream through the chamber(s), therebytransferring heat into the trough section through the outer surfacethereof. The apparatus preferably additionally comprises at least onegenerator of hot combustion gases, such as a fuel burner, positioned atthe entrance of the or each chamber.

Another exemplary embodiment provides a method of providing heat to amolten metal flowing through metal-conveying apparatus provided with atleast one channel for conveying said molten metal, an enclosure forreceiving and circulating combustion gases and a body of heat-conductivematerial separating at least part of said channel from said enclosure,said method comprising conveying molten metal through the channel,generating combustion gases, causing the combustion gases to enter andcirculate through the enclosure while confining said combustion gases toprevent said gases entering said channel.

Yet another exemplary embodiment provides a method of heating a sectionof a molten metal conveying trough having an upper end and an outersurface extending around the trough section from the upper end. Themethod comprises generating at least one stream of hot combustion gases,and directing the at least one stream to flow through an enclosed volumesurrounding at least part of the outer surface of the molten metalconveying trough section. The outer surface of the trough is therebyexposed to the stream of hot combustion gases, thereby causing heat totransfer to the trough section and its contents through the outersurface.

Preferably, the hot combustion gases are generated by a burner thatcreates a stream of hot gases and a flame introduced into the enclosure.The combustion gases are normally used directly, i.e. without having anopportunity to cool down to any significant extent. Ideally, the hotcombustion gases are preferably confined to follow a winding path whilein contact with the heat conductive refractory material and, ideally,substantially all of the surface of the heat conductive materialopposite to the metal-contacting surface is exposed to the hot gases.

The heat-conductive body may be made of any material that has sufficientheat conductivity to allow heat to pass at an effective rate from thehot combustion gases to the molten metal in the channel when used inthicknesses appropriate to provide good support for the molten metal anda robust apparatus. An “effective rate” of heat passage is, of course, arate sufficient to achieve the desired effect (e.g. molten metalheating, metal temperature retention, or slowed cooling of the metal asit passes through the channel). While any effective thickness ofmaterial may be used, thinner cross-sections are better because they areless resistant to the passage of heat, provided adequate strength isretained. The thickness selected is generally no greater than thatrequired for adequate strength of the trough section and good support ofthe molten metals. Normally, effective materials are used in thicknessesthat range from 0.25 inch to 12 inches or 0.5 inch to 6 inches, morepreferably 1 to 8 inches, and even more preferably 2 to 6 inches,depending on the type of material employed, although thinner or thickersections are not excluded. Of course, the thickness does not have to beconstant at all points in the material and thicknesses may vary frompoint to point, as required, as may the composition of the material.

Suitable heat conductive materials include, for example, refractorymetal compounds or solid metals. Many solid metals are attacked byflowing molten metal of the same or a different kind and are thereforenot suitable, unless the metal-contacting surface is protected in someway. Cast iron has been found to have a good resistance to attack bymolten metal (e.g. aluminum alloys) and the metal-contacting surface maybe further protected by applying a thin coating of a refractory metalcompound, e.g. boron nitride. Refractory metal compounds may be usedinstead of metal, provided they have good thermal conductivity or can beused in thin sections. Such materials are generally strong at hightemperatures, resistant to thermal shock, unreactive with molten metal,and have low coefficients of expansion. However, refractory metaloxides, e.g. alumina, silica and calcium oxide, are generally regardedas heat insulators may not be suitable (unless mixed with moreconductive materials or used in very thin sections) because they havelow thermal conductivity (e.g. usually less than about 2Watts/meter-Kelvin (W/mK). On the other hand, silicon carbide, boronnitride and silicon nitride are suitable materials (although boronnitride is extremely expensive, and is therefore unlikely to be used inpractice while its price remains so high).

It has been found that heat-conductive refractories containing siliconcarbide are particularly preferred, sometimes protected with a layer ofsilica to prevent oxidation at high temperatures. Although siliconcarbide may be used in its pure form, it is generally mixed in granularform with binders and other refractory compounds in water, cast, driedand cured to form a dense solid. The larger the proportion of siliconcarbide, the higher is the heat conductivity of the resultingrefractory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of an apparatus according to one embodiment ofthe present invention;

FIG. 2 is a side view of the apparatus of FIG. 1;

FIG. 3 is an end view of the apparatus of FIG. 1;

FIG. 4 is a vertical transverse cross section of the apparatus of FIG. 1taken on the line IV-IV shown in FIG. 2;

FIG. 5 is a horizontal cross section of the apparatus of FIG. 1 taken onthe line V-V shown in FIG. 2;

FIG. 6 is a vertical longitudinal cross section of the apparatus of FIG.1 taken on the line VI-VI shown in FIG. 1;

FIG. 7 is a central vertical longitudinal cross section of the apparatusof FIG. 1 taken on the line VII-VII shown in FIG. 1;

FIG. 8 is a cross-section similar to FIG. 5 of an alternative embodimentof the present invention;

FIG. 9 is a cross-section similar to that of FIG. 7, but showing amodified embodiment in which the trough has a constant depth throughoutits length, and an insulating cover over the open top;

FIG. 10 is a cross-section of an alternative exemplary embodiment; and

FIG. 11 is a cross-section of yet another alternative embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first exemplary embodiment of a molten metal conveying apparatus isshown in FIGS. 1 to 7 of the accompanying drawings. This particularembodiment is intended for use with metal degasser nozzles intended foruse with molten aluminum or aluminum alloys, thereby forming a compactin-line metal degasser unit which may, for example, be incorporated intoa conventional trough or launder leading from a metal melting furnace toa casting apparatus. Other exemplary embodiments may be intended for usewith other molten metals.

The apparatus is indicated generally by reference numeral 10 andincludes a section 11 of metal-conveying trough made up of two troughparts 12 and 13 abutting each other at a junction 14. The trough section11 acts as an elongated channel-forming element that conveys moltenmetal through the apparatus. Butting up to the trough section 11 at anupstream end is a trough inlet member 15, and butting up to the troughsection 11 at a downstream end is a trough outlet member 16. All ofthese parts are of generally U-shaped cross-section and are made of abody of heat-conductive refractory ceramic material, the same materialpreferably being used for all these parts. While most moltenmetal-conveying troughs are made of insulating refractories, e.g. metaloxides, designed to prevent undue cooling of the molten metal as it isconveyed through the trough, the trough section 11 is instead heatconductive. The preferred refractory material used for this purpose is adense cast ceramic having a high thermal conductivity made of orcontaining silicon carbide (SiC). This material is resistant to hightemperature and attack by aluminum and most aluminum alloys at theirnormal casting temperatures. The heat conductivity of such ceramicsincreases as the content of SiC increases and therefore it is desirableto use at least 25%, more preferably at least 50%, and even morepreferably at least 65% of SiC in the composition. Pure cast SiC may beused, but is expensive and somewhat brittle. A particularly preferredmaterial has the following composition:

SiC 80 wt. % Al₂O₃ 15 wt. % SiO₂  3 wt. % Balance miscellaneousrefractory components.

This material has a density of about 2.4 grams per cc and a heatconductivity in the range of 9.4 to 10.8 W/mK.

Arranged in the manner shown, inlet member 15, trough section 11 andoutlet member 16 are firmly held together, possibly under resilientlongitudinal compression provided by spring-loaded end plates (notshown), usually without any jointing or sealing compound, to form acontinuous elongated open-topped channel 17 for conveying molten metalin the direction of arrow A from one side of the apparatus to the other.Although not shown in the drawings, the inlet and outlet members are, inuse, joined to other metal-conveying apparatus or trough parts usingstandard means of attachment. In the illustrated embodiment, the inletmember 15 and outlet member 16 incorporate slight downward slopes fromtheir respective outer ends to their inner ends, thereby making thechannel 17 somewhat deeper within the trough section 11 than at theextreme inlet and outlet ends (see FIG. 7). However, it should be notedthat a deep section of this kind in a trough may make it difficult toremove all of the metal between metal-conveying operations, so thetrough section 11 and members 15 and 16 may alternatively be made ofconstant depth, if preferred.

As shown most clearly in FIG. 4, the trough section 11 has an upper end23, an inner surface 18 defining part of the channel 17 and an outersurface 19 forming flat side walls 20 and flat bottom wall 21 delimitingthe physical outer dimension of the trough section. Since the openchannel 17 occupies most of the upper end 23, there is virtually notrough outer surface at the upper end of the trough. The thickness ofthe channel section between the inner surface 18 and the outer surface19 is sufficient to confine and support the molten metal withoutyielding. It can be seen that the channel 17 formed within the troughsection 11 is relatively broad and deep so that it can hold quite alarge quantity of molten metal when sufficiently filled (e.g. up to thelevel 22 shown by broken line in FIG. 4). Of course, in otherembodiments, the channel section may be shaped and dimensioneddifferently to suit particular applications, and may be, for example,rectangular, V-shaped or semi-cylindrical. As already mentioned, theapparatus of this exemplary embodiment is intended to be used with metaldegassers (e.g. spinning gas injectors, one of which 25 is representedin broken lines in FIG. 4), and the deep and broad shape and dimensionof the channel 17 allows sufficient room for the immersion and use ofsuch degassers, a good head of metal above the gas insertion points(which improves metal degassing and cleaning operations), and optionallya relative slow rate of metal flow through the channel of about 3meters/minute or less (in other applications where the channel sectionis used primarily for metal delivery, a higher flow rate of 4 to 9meters/minute is more common and preferred). The particularcross-sectional shape and size of the channel 17 also means that theratio of the molten metal surface 22 exposed to the atmosphere relativeto the volume of contained molten metal is quite small, so surfaceoxidation does not present as much of a problem as would be the case fora shallower or wider trough. A cover (not shown in this embodiment, butsee element 60 of FIG. 9) may be positioned over the channel 17 toreduce heat loss from the molten metal, although, in this embodiment,such a cover (if used) requires holes to allow the gas injectors 25 topass through.

As can be seen from the drawings, the trough section 11 is surrounded onall sides, except at the open top 26 of the channel 17, by an enclosurein the form of a housing 30 comprising a metal-sided tank 31 lined withheat insulating refractory material 32 made, for example, of fire bricksstacked side-by-side or one on top of another, optionally without anyjointing or sealing compounds, although a refractory mortar may be usedbetween the bricks, if desired. The open top of the tank is wider thanthe trough section 11 and the gaps between the sides of the tank and theupper edges of the trough section are also bridged and closed off byrefractory blocks 33, e.g. ceramic bricks laid transversely relative tothe long dimension of the trough section and supported at their inneredges by notched corners of the trough section itself at the upper end23, as shown in FIG. 4. Removable insulating covers 34 are positionedover the refractory lining material 33 to provide a relatively coolupper surface for the safety of operators. Within the housing 30, thetrough section 11 is supported by a short vertical wall 35 risingupwardly from the floor of the tank along the longitudinal centerline,and also by a vertical wall 36 extending transversely of the troughsection (see FIG. 5 in particular). The junction 14 of the two parts 12and 13 of the trough section is aligned with wall 36 to prevent slippagebetween the parts. Any tendency of the trough section 11 to sag or slipunder the weight of the metal at the operating temperature is thusavoided by the underlying rigid and effective support provided by walls35 and 36.

As shown in FIG. 3, the outlet member 16 of the trough is retainedwithin an open-topped metal shell 37 held in place by an open-toppedrectangular bracket 38 bolted to end wall 39 of the tank. A similararrangement is provided for the inlet member 15 of the trough at theother end of the apparatus.

As can be seen best from the horizontal cross-section of FIG. 5, theinterior of the housing 30 incorporates two hollow chambers 40 formingenclosed spaces isolated from the outside atmosphere and mutuallyaligned with one following the other in the longitudinal direction ofthe apparatus. The chambers 40 are separated by transverse wall 36 whichextends closely around the outer surface 19 of the trough section 11(see FIG. 6) and thereby isolates the atmospheres within the twochambers from each other. Each chamber 40 is divided down the center bylongitudinal wall 35 to form two hollow compartments 41, but thesecompartments communicate with each other by virtue of the fact that thelongitudinal walls 35 do not extend fully up to the transverse wall 36,leaving gaps 43 on each side.

It is preferable that substantially all of the outer surface 19 of thetrough section 11 should be encircled by the housing and chambers, i.e.at least those parts of the trough between end walls 39 and 45 of thehousing. In some embodiments, however, it may be possible to encloseless of the outer surface of the trough, i.e. the upper end of thetrough may stand clear of the housing, or the bottom wall of the troughmay rest on the bottom of the tank and may not be exposed to theinternal chambers 40. Generally, however, at least 50%, and morepreferably at least 75%, and optionally at least 95% of the outersurface of the trough section is enclosed and encircled by the internalcompartments and chambers, thereby ensuring (as will be explained) goodand even heat delivery to the trough section and molten metal containedtherein. Any parts of the trough that are not enclosed in this way may,if desired, be covered by a layer of heat insulating material to preventundue heat loss from these parts.

As already noted, in the illustrated embodiment, substantially theentire outer surface 19 of the trough section 11 is surrounded by anddirectly exposed to the internal compartments 41 of each chamber 40,i.e. not only at the sides 20 but also along the bottom 21. The onlyparts of the trough section not directly exposed to these hollowcompartments are the parts supported by the walls 35 and 36, and theparts in contact with the refractory material 33 at the top edges. Theseparts of the trough section add up to only a small percentage of theouter wall of the trough section. Two openings 46 and 47 are formed ineach of the end walls 39 and 45 of the tank 31 and pass through theadjacent refractory lining. Openings 46 are intended as inlets for hotcombustion gases into the respective chambers 40, and openings 47 areintended as outlets for such gases (and are normally each connected togas exhaust piping, not shown). Fuel burners 50 are positioned in oradjacent to the inlet openings 46 to generate streams of hot combustiongases, and optionally flames 51, and to introduce them into thecompartments 41, as shown in FIG. 5. The hot gases circulate between thecompartments 41 in each of the chambers 40 by virtue of the gaps 43positioned at a distance from each of the inlets and outlets. Thiscirculation of hot gases is represented by arrows B. The gaseseventually leave the apparatus via the outlets 47, as represented byarrows C. As shown in FIG. 6, the hot gases are free to ascend along thesides of the trough section 11, as shown by arrows D, so that thesubstantially the entire outer surface 19 of the trough section 11 isexposed to and bathed in the hot circulating combustion gases duringoperation of the burners. Collectively, the movements represented byarrows B, C and D form a steady stream of hot gases flowing through thechambers 40. It will be noted that the chambers are completely enclosedwithin the housing 30 and are sealed against loss of gases, except atinlets and outlets 46 and 47, so the streams of hot combustion gases areconstrained to follow a winding or sinuous or serpentine path througheach chamber, i.e. from compartment to compartment 41 via the distantgap 43 with the gases flowing in opposite directions in eachcompartment. It will be understood that the combustion gases arechanneled and constrained in such a way that they are prevented fromentering the channel 17 and coming into contact with the molten metalconveyed through the apparatus.

In practice, trough section 11 is heated at its outer surface 19 by bothradiant heat from the flames 51 and conduction/convection from directcontact with the hot combustion gases. The relatively good heatconductivity of the material of the trough section 11 allows the heat topenetrate through the trough section and into the channel 17 and moltenmetal held therein. The openings 46 and 47, and the burners 50, arepreferably positioned and angled such that flames 51 and the stream ofhot gases are not initially directed onto the outer surface 19 of thetrough section 11, nor onto the refractory lining 32, 33, therebyavoiding the formation of hot-spots and possible damage to therefractory surfaces. The flame and hot gases from the burners 50 aregenerally oriented horizontally in the longitudinal direction of thetrough section into an open area of each chamber 40 beneath the level ofthe bottom wall of the trough section. This arrangement also ensuresgood heat distribution across the entire outer surface 19 of the troughsection and thus prevents the formation of cool spots within the metalchannel 17. It will be noted that the hot gases passing through thechambers 40 encounter only the refractory of the tank lining or therefractory of the trough section so that the high temperatures areaccommodated without damage to the apparatus or undue heat loss.

The burners 50, which may for example be gas-fired or oil-fired, areprovided with suitable heating capacity to raise the temperature in thechambers quickly and to introduce sufficient heat into the troughsection 11 to raise the temperature of the molten metal in channel 17,to keep the temperature of the molten metal constant, or to allow themolten metal to cool in a controlled manner, depending on the plans forthe metal. Examples of suitable burners are so-called premix burnersthat are aspirated at the spud and the burner throat created by thevelocity of gas moving through a nozzle. The mix of fuel and air and maybe controlled by a manual valve, or may be controlled automatically,e.g. by a computer following a pre-determined program. Examples of suchburners are disclosed in the North American Combustion Handbook (1978),North American Mfg. Co., Second Edition, 1978 (ISBN: 0960159614), page243, FIGS. 6.7 (inspirator design) and 6.8 (aspirator design). Thedisclosure of this handbook is specifically incorporated herein by thisreference. As an alternative, compressed air may be used to jet thecombustion gases into the chambers 40, or a nozzle-mix burner may beused in which the burner mixes air and gas but requires a blower toprovide the air. In all cases, there is a necessary fuel supply withappropriate safety equipment to control purging, pressure, flamemonitor, etc. Generally, the combustion gases have a temperature in therange of 500 to 2000° C. or more when introduced into the apparatus, andare thus capable of delivering heat rapidly and in unlimited quantities.

In operation, if desired, the ceramic material of the trough section maybe raised quickly to a suitable high temperature by the burners when theapparatus is first put into operation, and such temperature can bemaintained indefinitely during normal metal flow. Alternatively, theapparatus may be heated by the combustion gases before metal is causedto flow through the trough, thereby avoiding rapid cooling of the metalas the first flow of the hot metal pours into the apparatus. Once asteady temperature has been reached, the output of the fuel burners 50may be scaled back or cycled on and off to maintain an equilibriumtemperature under the control of thermocouples or similartemperature-sensing devices, ideally monitored by computer numericalequipment. For this purpose, two thermocouples 55, 56 are provided tocontrol the temperature of each chamber, one (55) for the control of thetemperature of the trough and/or the metal within the trough, and theother (56) for control of over-temperature within the chamber. Troughtemperature is taken outside the fire box near to the burner viathermocouple 55 positioned in direct contact with the trough.Alternatively, a thermocouple may be provided in contact with the moltenmetal and extending into the trough from the open upper end. The secondthermocouple 56 is positioned in contact with the refractory 32 in thecoolest part of the chamber. The burner 50 is then cycled between twocontrol points, i.e. low metal temperature cycles the burner on, andhigh chamber temperature cycles the burner off. Backup thermocouples 57are also provided in case of failure of the primary thermocouples.

Thermocouples may be provided only on one long side of the housing 30,but may alternatively be positioned on both sides. In general, thethermocouples are provided on the burner side of a chamber, but theburner of each chamber may be positioned differently in differentinstallations due to such considerations as available space and exhaustfacilities, etc., so it is prudent to provide thermocouples on each sideduring the production of the apparatus. Also, it should be kept in mindthat in a two-chamber housing of the kind shown in the FIGS. 1 to 7, theburners of one chamber may be positioned on the opposite transverse sidefrom the burner of the other in contrast to the same-sided arrangementshown in the drawings. Indeed, this may be preferred for evendistribution of heat along the trough section.

It will be noted from the drawings that there is no barrier or layer ofmaterial of any kind between the outer surface 19 of the trough section11 and the inside of the chambers 40, because any such barrier or layerwould add a measure of insulation between the trough section and the hotcombustion gases, thereby slowing the temperature response of theapparatus or reducing the maximum temperature that may be imparted tothe molten metal. However, a thin covering or shell of material, such asmetal or protective ceramic layer, may be provided to support andprotect the material of the trough section, if this is considereddesirable. Such a layer should preferably be thin enough (or conductiveenough) to provide little or no heat-insulation value.

The burners 50 are fed with fuel through conventional oil or gas lines(not shown in the drawings) and the lines may be secured by a hose clamp58 as shown in FIG. 3. In FIG. 4, the position of inlet 46 and outlet 47are shown in broken lines to indicate their positions relative to theinterior, although it will be realized that these elements are formed inthe exterior wall (not shown in FIG. 4).

FIGS. 1 to 7 represent an exemplary embodiment in which there are twolongitudinal heating chambers 40 within the housing, each divided intotwo lateral compartments 41, which is an arrangement that is normallypreferred. However, for a relatively short trough section, there may bejust a single chamber with two compartments, one inlet, one outlet andone fuel burner (the inlet and outlet being positioned in the same sidewall, and the chamber extending for the full length of the troughsection). For a longer trough section, more than two chambers may beprovided. For example, FIG. 8 is a view similar to FIG. 5, but showing athree-chamber apparatus. In this case, an additional chamber 40′ ispositioned between two end chambers 40. The additional chamber has adivider wall 36′ that divides the chamber into two compartments 41′ andforces the hot combustion gases entering the chamber 40′ through sideinlet 46′ from burner 50′ to extend around the end of the divider wallas shown by arrows B′ before emerging from the compartment at sideoutlet 47′. Additional similar chambers may be provided, if required. Itis to be noted that the provision of more burners and chambers makes itpossible to introduce greater amounts of heat into the apparatus andoffers a more precise control of temperature or temperature profilealong the channel.

As noted earlier, the apparatus of FIGS. 1-7 (and also FIG. 8) isintended to provide a trough section suitable for use with metaldegasser nozzles and is therefore quite deep. FIG. 9 shows analternative embodiment having a shallower trough section 11 intended formore general use for conveying molten metal from one location toanother. In this case, the floor of the trough section 11 is flatthroughout its length and there are no trough inlet and outlet membersas in the earlier apparatus. The overall height of the trough section 11should preferably be approximately 100 mm above the metal level 22 forsafety. As there is no intention to introduce devices such as gasnozzles into the metal in this form of the apparatus, an insulatingcover 60 (either removable or fixed) may be positioned over the openupper end of the trough section to provide heat insulation for themolten metal.

In the case of a two-chamber apparatus of FIGS. 1 to 7, the length ofthe trough section is normally about 6.5 ft. and the two burnerscombined are capable of generating a maximum of at least 600,000 Btu/hr,or 92,000 Btu/hr/ft during apparatus heat-up (for a total of 600,000Btu/hr). In steady-state operation, the output of the burners may bescaled back to about 360,000 Btu/hr, or 55,000 Btu/hr/ft. When gasfired, the burners may consume 12,000 liters per minute of gas atmaximum output. The amount of air supplied to the burners should be anamount suitable for complete combustion of the gas to carbon dioxide(usually an excess of 3% over the stoichiometrical amount required forcomplete combustion), e.g. 120,000 liters per minute. This degree ofheating ideally keeps the metal within a suitable temperature range,e.g. 20° C. above the liquidus (or a minimum of 350° C.) up to 1300° C.(for aluminum and aluminum alloys), and up to about 850° C., or even upto about 1000° C. A particularly preferred range is 650-725° C. It is tobe noted that a large amount of the heating effect may be brought aboutby radiant heating as well as convection heating.

The metal movement through the trough section is generally expressed interms of mass flow. The preferred rate is 86-550 lbs/minute, or about2-5 cm/sec, although there is really no lower limit as the metal may bekept molten even when it is stationary. Generally, the flow should notbe so fast that it becomes turbulent, which often occurs within therange of 15-20 cm/sec.

If necessary, when the apparatus of the illustrated embodiments isattached to other trough sections, those sections (particularly ifshallower) may also be heated, but by other means, e.g. by electricalheaters embedded in the trough walls or used to produce radiant heatfrom above.

While the previous exemplary embodiments incorporate open-topped troughsections made of heat conductive refractory material, other arrangementsmay be provided. For example, further alternative exemplary embodimentsof the invention are shown in FIGS. 10 and 11. In the embodiment of FIG.10, molten metal 22 is conveyed through six parallel tubes 111 made of aheat conductive material, preferably containing silicon carbide. Thetubes have inner, metal-contacting, surfaces 118 and outer surfaces 119that remain out of contact with the molten metal. The tubes aresurrounded by an enclosure 132 made of an insulating refractorymaterial, e.g. a material made of refractory metal oxides. The enclosedspace between the exterior of the tubes 111 and the interior of theenclosure 132 forms a passage 141 through which hot combustion gases arecaused to flow and circulate (e.g. a burner is provided at an inlet atone longitudinal end of the enclosure 132 and a vent for the gases isproved at an opposite longitudinal end). The metal within the tubes 111is kept hot by heat from the combustion gases passing through the wallsof the tubes 111, whereas heat is retaining within the apparatus 110 bythe insulation provided by the enclosure 132. The hot combustion gasesin the channels formed by the tubes do not contact the molten metal asthe gases are confined to follow a separate path and are vented beforethe molten metal exits the apparatus.

In the embodiment of FIG. 11, molten metal 22 is conveyed through anelongated trough 250 made of a heat insulating material, e.g. a materialmade of refractory metal oxides. Suspended within the molten metal 22 isa body 211 of heat conductive material, preferably a refractorysubstance made of or containing silicon carbide. The body is fabricatedin the form of a hollow tubular element encircling an enclosed space240. The body 211 has an outer surface 218 that contact the molten metalin the trough, and an inner surface 219 that is out of contact with themetal. Hot combustion gases are caused to flow through the enclosedspace 240, e.g. by providing a burner at an inlet at one longitudinalend of the body 211 and a vent at an opposite longitudinal end. The body211 consequently confines and circulates the hot gases and keeps thegases out of contact with the molten metal in the trough 250. The moltenmetal is kept hot by heat from the combustion gases that passes throughthe conductive walls of the body 211. A removable cover 260 is providedto reduce heat losses from the surface of the molten metal.

COMPARATIVE INFORMATION

Potential materials for the heated trough were investigated for thermalconductivity and resistance to attack by molten aluminum. The resultsare shown in the Table 1 below.

TABLE 1 Thermal Resistance Density Conductivity to Molten SupplierProduct Composition g/cc W/mK Al Notes Pyrotek O'Sialon 65% SiC 2.6 9 OKAndeman EC70P 70% SiC, Al 2.1 7 OK silicate Pyrotek Pyrocast 77% SiC, Al2.6 7 OK SCM2600 silicate Pyrotek Pyrocast 83% SiC, 2.4 10 OK SC2600alumina Andeman EC90P 90% SiC, Al 2.2 25 OK silicate Aremco BisqueAlumina 2.8 4 Cracks Machinable Fired Alumina Pyrotek Pyrocast Alumina -2.7 6 OK Contains metal ZA Metal fibers for strength St. AX05 BN 1.8 78OK Very expensive Gobain GE BNC1 BN 2.2 10 OK Machinable CompositePyrotek Pyrocast Fused SiO, 2.3 1-2 OK ZR Al silicate SGL EK10 Graphite1.7 10 OK Burns in air Carbon Morgan Frequentite Mg Silicate 2.8 3 Nodata 1000 Ceradyne 147-1B Si₃N₄ 2.3 14 OK Machinable before filingEKatherm Si₃N₄ 3.2 22 OK Machinable before filing Plain C 7.9 50Dissolves steel Alloy steel 7.8 40 Dissolves Stainless 7.9 15 Dissolvessteel Hi-Ni 7.4 13 Dissolves ductile iron Cast Iron 80 Erodes Can becoated with a wash (e.g. BN) to extend service life

A review of the published properties of various forms of SiC revealedthe information shown in Table 2 below (from MatWeb website).

TABLE 2 Thermal Conductivity Material W/mK SiC, sintered alpha 126 SiC,sublimed 110 SiC, 99.9995% 200 SiC, hot pressed 70 SiC, zero porosity100-160 SiC, reaction bonded 125 SiC, sintered 150 SiC, Chemical VaporDeposition 99.9995% 115 SiC, fibers 150 SiC, synthetic 90 SiC, beta 42

It appears that all these forms of SiC are of very high thermalconductivity, and may thus be used in the illustrated embodiments whensufficiently strong and durable.

From these tables, it can be seen that a preferred range of thermalconductivity is at least about 2.5 W/mK, e.g. in the range of about 2.5to 200 W/mK, with more preferred ranges being 5 to 80 W/mK and 7 to 25W/mK.

1. A molten metal-conveying apparatus, comprising: a moltenmetal-conveying channel; an enclosure for receiving and circulatingcombustion gases while preventing entry of said gases into said channel;a heat-conductive body of material separating at least part of saidchannel from said enclosure; and a combustion device for generatingcombustion gases and delivering said gases to said enclosure; whereby,in use, heat from said combustion gases is transferred to molten metalin said channel through said body of heat conductive material.
 2. Theapparatus of claim 1, wherein said material has a thermal conductivityin a range of 2.5 to 200 W/mK.
 3. The apparatus of claim 1, wherein saidmaterial has a thermal conductivity in a range of 5 to 80 W/mK.
 4. Theapparatus of claim 1, wherein said material has a thermal conductivityof 7 to 25 W/mK.
 5. The apparatus of claim 1, wherein said materialcomprises a refractory metal compound.
 6. The apparatus of claim 5,wherein said refractory metal compound is selected from the groupconsisting of silicon carbide, boron nitride and silicon nitride.
 7. Theapparatus of claim 5, wherein said material comprises at least 65% byweight of silicon carbide.
 8. The apparatus of claim 1, wherein saidmaterial is a metal having a coating of a substance, at least on ametal-contacting surface of the material, that is resistant to attack bysaid molten metal.
 9. The apparatus of claim 8, wherein said metal iscast iron.
 10. The apparatus of claim 8, wherein the substance is boronnitride.
 11. The apparatus of claim 1, wherein said channel is definedby an open-topped trough section and said enclosure encircles an outersurface of said trough section.
 12. The apparatus of claim 1, whereinchannel is defined by at least one tube adapted to convey molten metaltherethrough, said body of material forms walls of said at least onetube, and said enclosure fully surrounding said at least one tube. 13.The apparatus of claim 1, wherein said body of material forms a hollowtubular element suspended in said channel, and said hollow tubularelement acts as said enclosure defining said enclosed space within saidelement.
 14. A molten metal-conveying trough apparatus, comprising: amolten metal conveying trough section having an upper end and an outersurface extending around the trough section from said upper end; anenclosure at least partially enclosing said outer surface of the troughsection, said enclosure containing at least one enclosed chamberadjacent to said outer surface; an entrance into the chamber, or anentrance into each chamber when more than one, through which hotcombustion gases can be introduced into the or each chamber; and an exitfrom the chamber, or each chamber when more than one, through which saidhot combustion gases can be removed from the chamber, or each chamberwhen more than one, after flowing through the or each chamber, therebytransferring heat to the trough section through said outer surface. 15.The apparatus of claim 14, wherein said enclosure encloses substantiallyall of said outer surface of the trough section.
 16. The apparatus ofclaim 14, further comprising at least one generator of a stream of hotcombustion gases, one said generator being positioned at said entranceof the or each chamber.
 17. The apparatus of claim 16, wherein said atleast one generator introduces said stream of hot combustion gases intothe or each chamber generally horizontally beneath said trough section.18. The apparatus of claim 14, wherein said trough section is made of aheat-conductive refractory material.
 19. The apparatus of claim 14,wherein the heat-conductive refractory material comprise siliconcarbide.
 20. The apparatus of claim 14, having at least two saidchambers arranged one following another in a longitudinal direction ofsaid trough section.
 21. The apparatus of claim 14, wherein the or eachchamber comprises at least two compartments interconnected together at adistance from said inlet and said outlet and positioned to confine saidstream of hot combustion gases to flow along an extended path adjacentto said outer wall of said trough section.
 22. The apparatus of claim14, wherein the chamber, or each chamber when more than one, has aninner volume in use receiving said hot combustion gases, and said outersurface of said trough section is directly exposed to said inner volume.23. The apparatus of claim 14, further comprising at least twothermocouples, one positioned to measure temperatures of molten metalwhen present in said trough section, and another positioned to measuretemperatures in said chamber, or at least one of said chambers when morethan one.
 24. The apparatus of claim 14, further comprising aheat-insulating cover positioned over said upper end of said troughsection.
 25. A method of providing heat to a molten metal flowingthrough metal-conveying apparatus provided with at least one channel forconveying said molten metal, an enclosure for receiving and circulatingcombustion gases and a heat-conductive body of material separating atleast part of said channel from said enclosure, said method comprising:conveying molten metal through said channel; generating combustiongases; causing said combustion gases to enter and circulate through saidenclosure while confining said combustion gases to prevent said gasesentering said channel.
 26. A method of heating a section of a moltenmetal conveying trough having an upper end and an outer surfaceextending around the trough section from said upper end, whichcomprises: generating at least one moving stream of hot combustiongases; and directing said at least one stream of hot combustion gases toflow through at least one enclosed volume surrounding at least part ofsaid outer surface of said molten metal conveying trough section,thereby exposing said at least a part of said outer surface of saidtrough section to said hot combustion gases and enabling heat totransfer into said trough section through said outer surface.
 27. Themethod of claim 26, wherein said at least one enclosed volume surroundssubstantially all of said outer surface of said trough section.
 28. Themethod of claim 27, wherein said at least one stream is directed to flowin an extended path adjacent to said outer surface of said troughsection.
 29. The method of claim 28, wherein said at least one stream isdirected to flow in an extended winding path.
 30. The method of claim26, wherein said stream of hot combustion gases is generated by burningfuel in a stream of combustion air.
 31. The method of claim 26, whereinsaid at least one stream of gases is directed to flow initially beneathsaid trough section.
 32. The method of claim 26, wherein at least twostreams of said hot combustion gases are generated and each is directedto flow through a different enclosed volume, each volume being arrangedone after another in a longitudinal direction of said trough section.